Acutely transforming RNA tumor viruses contain genes (oncogenes) derived from normal cellular DNA. Several of these cellular protooncogenes have been shown to be growth factor receptors, or growth factors themselves. Thus, it has been hypothesized that the uncontrolled growth exhibited by cells transformed by these viruses is due to auto-stimulation of a growth factor pathway. The feline virus SM-FeSV contains a gene designated fms which has been shown to be related (or identical) to the gene for macrophage colony stimulating factor (M-CSF). Cells transformed by the virus synthesize elevated levels of a surface glycoprotein, which is phosphorylated in vitro on tyrosine. The amino half of the protein is exposed at the cell surface, and is presumed to be the domain which interacts which M-CSF. A 25-amino acid membrane anchor region separates the ligand-binding domain from the cytoplasmic domain, which includes a region related to the tyrosine protein kinase family of oncogene products. The normal c-fms gene is expressed on mature macrophages, and on promonocytic cells induced to differentiate to macrophages. It is also expressed during embryonic development at elevated levels in the placenta. The studies will focus on two aspects of the fms protooncogene. (1) The steps required to convert a normal c-fms gene to a transforming one will be investigated. This will include an analysis of the significance of the altered amino terminal and carboxy terminal halves of the viral gene product by exchanges between the clones of the viral fms gene and a c-fms cDNA. We will also examine the importance of a 70-amino acid, fms-unique segment within the protein kinase region. The segment may be important for transformation, or may be essential only for normal function of the cellular fms gene. (2) The role played by the c-fms gene in normal cellular development will be studied. The requirement for the c-fms gene for conversion of promonocytes to macrophages will be examined by generating anti-sense fms RNA, under control of an inducible promoter, to interfere with normal translation of the gene. We will also use in situ hybridization and immunocytolocalization to determine whether the fms gene is expressed in extraembryonal tissues per se, or in the macrophages found in these tissues enriched for cells of the hematopoetic lineage.